1. Field of the Invention
The invention relates to a semiconductor memory device.
2. Related Art
FBC memory devices are semiconductor memory devices expected recently as alternatives to conventional DRAMs. In the FBC memory device, an FET (Field Effect Transistor) with a floating body (also referred to as body region) is formed on an SOI (Silicon On Insulator) substrate. Data “1” or “0” is stored depending on the number of majority carriers accumulated in the body region.
Meanwhile, SOONO (Silicon-On-ONO) MOSFETs have been disclosed (C. W. Oh et. al., “A Novel Multi-Functional Silicon-On-ONO(SOONO) MOSFETs for SoC Applications: Electrical Characterization for High Performance Transistor and Embedded Memory Applications”, 2006 Symposium on VLSI Technology Digest of Technical Papers, pp. 58-59, hereinafter referred to as non-patent document 1). The SOONO MOSFET is an MOS transistor that an ONO film is used instead of a BOX (Buried Oxide) layer of a conventional SOI MOSFET. The non-patent document 1 describes the usual DRAM function using the SOONO MOSFET and the function as a non-volatile memory using the SOONO MOSFET. If the SOONO MOSFET is used as the non-volatile memory, charges are trapped in the ONO film. The threshold voltage of the MOSFET is varied depending on the amount of charges trapped in the ONO film. Data is distinguished by the variation in threshold voltage.
If the SOONO MOSFET is used as the non-volatile memory, however, it takes 500 microseconds (μs) to program and erase data. The time is longer than a time required for programming and erasing data in a DRAM (e.g., 30 nanoseconds (ns)). A memory cell that charges are trapped in the ONO (Oxide-Nitride-Oxide) film is used only for a ROM (Read Only Memory).